This invention relates to an aspirator for use in conjunction with a pneumatic thermostat or the like. More particularly this invention relates to a novel restriction for use within such an aspirator.
In a temperature control system having a plurality of thermostats it is occasionally desirable to have one or more thermostats tamper-proof or protected from accidental damage. This is usually accomplished by mounting the thermostat within a wall box in a recess in the wall, and covering the thermostat with a coverplate which is substantially flush with the wall surface. The thermostat, when so mounted, tends to sense and control primarily the temperature of the wall. It is desired, of course, that the thermostat control the temperature of the entire space. In order to overcome this problem ambient air is pumped from the space into the wall box where the ambient air temperature is sensed. In a pneumatic temperature control system this pumping action is usually provided by an aspirator.
The principle of operation of an aspirator is quite simple. A high velocity jet of air, emitted from a nozzle or restriction, tends to entrain additional air in the vicinity of the nozzle due to the low pressure associated with the high velocity stream. The aspirator utilizes this effect to pump ambient air past the thermostat. The nozzle or restriction of the aspirator is arranged to direct a high velocity jet of air towards an outlet slot or louver in the coverplate. Air within the wall box in the vicinity of the thermostat is entrained by this stream and is exhausted out through the slot in the coverplate. Accordingly air is drawn into the wall box through an inlet slot or louver in the coverplate, thereby providing a continuous stream of ambient air past the thermostat.
Two characteristics of an aspirator are considered in determining its efficiency: the volume of primary air consumed by the aspirator; and the volume of secondary or ambient air entrained and circulated past the thermostat. It is desirable to minimize the former to keep system operation costs down, and to maximize the latter to ensure accurate space temperature control. A measure of the efficiency of an aspirator is given by the ratio of the ambient air entrained to the primary air consumed.
Prior art devices have generally utilized a simple annular nozzle connected in series with a restriction for providing the high velocity jet. The nozzle is merely aimed at the outlet in the coverplate as is shown in Wehlau U.S. Pat. No. 3,232,112. Such arrangements, however, are relatively inefficient. This low efficiency may result either in high primary air consumption or in the thermostat being unduly influenced by a hot or cold wall. For example, if the wall in which the thermostat is located is cold, the thermostat will in effect try to raise the wall temperature. The likely result is an uncomfortably high space temperature.
The subject invention provides a high efficiency aspirator, i.e. one having high ambient air flow while maintaining low primary air consumption, by utilizing a slit or shallow channel restriction. A relatively wide and thin jet of air is emitted by the aspirator and directed towards the outlet slot or louver in the coverplate. The slit restriction provides a jet of air having a substantially increased jet surface area as seen by ambient air within the wall box as opposed to the prior art annular restriction. This increased surface area results in significantly more ambient air being entrained by the jet and circulated past the thermostat without any increase in primary air consumption. The thermostat used in combination with an aspirator having a slit restriction will thus more accurately control the temperature of the space and will be influenced significantly less by a hot or cold wall.
A further advantage of the subject invention resides in the combined use of the slit restriction as both a restriction and a nozzle. Most prior art aspirators have used a restriction connected in series with an annular nozzle.